In recent years, such wireless instruments for receiving radio frequency signals with an antenna system embedded in the wristband have become common. Many prior art solutions disclose an antenna device having a circumferentially variable size, embedded in a wristband, for use with a radio that is worn on the arm of a person. By doing this, the antenna can be made long enough to receive frequency signals beyond the VHF band (30–300 MHz). As shown on FIG. 3, the loop antenna 101 can be formed in a unitary fashion inside the wristband 102, which is connected to the casing 103 of the wrist-carried wireless instrument 100 to form a continuous loop via a center fastening structure 104, for example a clasp, of the wristband when the band is fastened.
However, in such arrangements the loop connection at the center fastening structure 104 significantly influences reception. Consequently it is difficult to design a mechanism that provides favourable operation, as this part is prone to break down. In addition, the wristband 102 typically contains a wristband adjusting structure to adjust the length of the wristband to the thickness of the wearer's arm. This adjustment causes the antenna's loop length to vary from wearer to wearer, which causes variations in the receivable frequency band from one wearer to another.
A solution consisting in providing the wireless instrument 100 with an additional apparatus for compensating changes in antenna gain and resonance frequency resulting from changes in the antenna's loop length is complex and bulky, which is not desirable in such wireless instruments.
According to the U.S. Pat. No. 5,986,566, it is disclosed a solution, shown on FIG. 4, to prevent connection failure and/or breakdown due to attachment or detachment of a loop antenna, of the afore cited type, and to provide a wrist-carried wireless instrument whose receivable frequency band is not affected by the thickness of the wearer's arm.
The wrist-carried wireless instrument 110 includes a casing 113, a center fastening-type wristband 112. The wristband 112 has upper 121 and lower 122 surfaces and a fastening structure 114 at its center and consists of a pair of wristband parts 112a and 112b, each of which is attached to an end of the casing 113. A receiving antenna 111 is mounted inside in at least one part 112a of the wristband to receive signals, the antenna 111 being connected via terminals to a reception circuit inside the casing 113. According to this document, the loop antenna 111 extends between the upper 121 and lower 122 surfaces of the wristband 112 and does not go through the center fastening structure 114. It is to be noted that reception would be possible without having the wristband 112 attached and forming a loop, as it does when worn.
Nevertheless, the solution according to the U.S. Pat. No. 5,986,566 has some drawbacks. Measures done on the antenna structure described hereinbefore show non-optimum antenna efficiency as well as non-optimum noise matching. A non-negligible part of the antenna losses is due to the human wrist interfering with the antenna and also due to the dielectric material of the wristband. Antenna efficiency improvements can be achieved by reducing the ratio of the antenna losses over the antenna radiation resistance. Therefore, antenna efficiency can be improved either by reducing the antenna losses or by increasing the antenna radiation resistance.
It is then an object of the present invention to improve antenna efficiency by reducing ohmic and dielectric losses of the antenna and increasing the antenna radiation resistance.